This invention relates to thin film semiconductor devices and, more particularly, to thin film devices having an interfacial metallic layer associated with one electrode to minimize shunting at pinhole-type defects of an active layer.
Thin film devices, and specifically photovoltaic devices, are often deposited on glass substrates having metal oxide coatings which serve as transparent "window" electrodes. A semiconductor layer containing a rectifying junction is deposited onto the window layer, followed by a back electrode to complete the device. Unfortunately, many thin film semiconductors tend to be deposited nonuniformly, causing pinholes, voids, thin spots or other structural defects (hereinafter collectively referred to as "pinhole-type defects") which join the two electrodes. Such defects form electrically conductive paths which can shunt the device and render it useless.
Various methods have been tried to reduce pinhole-type defects, or to fix the resulting shunts once they have been formed, but such methods have met with only limited success. In the case of thin film silicon photovoltaic cells having a window electrode of tin oxide and a back contact electrode of aluminum, applying a potential opposite to the usual polarity of the device has been observed to produce flashes of light visible through the window electrodes. Tests have shown that photovoltaic devices processed in this manner have improved operating efficiency. This indicates that at least some of the pinhole-type defects have been eliminated.
Unfortunately, not all thin film device structures can be "fixed" by applying a reverse bias potential. When known electrode materials other than tin oxide and aluminum are used, a reverse bias voltage often has little or no effect on the existence of shunts. This is the case when zinc oxide is used as a window electrode in a device having a thin film silicon-containing layer. Such devices exhibit significant shunting and no fixing effect when the second electrode is selected from metals heretofore proposed.
Therefore, it is desirable in many applications to provide a device and a method of manufacturing the device which minimize shunts at pinhole-type defects of a silicon-containing semiconductor layer and facilitate fixing of such shunts.